This invention pertains to key-operated electronic musical instruments, and more specifically to a system or circuitry responsive to the depression speeds of individual keys on such an instrument for controlling the tone production manner of corresponding tones in conformity with the speeds at which the keys have been depressed.
As heretofore incorporated in a keyboard-type electronic musical instrument, a typically touch-responsive volume control system is shown in FIG. 1 in which a first capacitor 2 is charged by a voltage supply +V.sub.1 through a key switch 1 while the key corresponding thereto is not depressed. The charge on the capacitor 2 is released to ground through a resistor 3 when the movable contact of the key switch 1 moves out of engagement with its break contact upon depression of the key. When the movable contact of the key switch subsequently comes into engagement with its make contact, the remaining voltage of the capacitor 2 is used to cause conduction through a transistor 4. Thereupon a voltage supply +V.sub.2 charges a second capacitor 5 to an extent corresponding to the remaining voltage of the first capacitor 2. As the second capacitor 5 subsequently discharges through a resistor 6, there is obtained an envelope control signal, as depicted in the figure, which has an amplitude corresponding to the key depression speed. This envelope signal is applied to a gating circuit 7 to control the amplitude of a tone source signal, corresponding to the depressed key, from a tone source circuit 8.
As seen from the above, the known system demands the use of two capacitors for each key of the instrument. Since the current integrated circuit (IC) fabrication technology does not allow integration of capacitors of this class into a microminiature circuit, the IC for touch-responsive control of an electronic musical instrument has required the provision of a multiplicity of leads for connection to the external capacitors. Further, the capacitors requires a large space on a circuit board. Therefore, it is desirable to reduce the number of capacitors to an absolute minimum.
It is also known from the Japanese Utility Model Publication No. Sho 48-42932 to provide a one-capacitor type touch-responsive control for an electronic musical instrument, in which the capacitor is charged during key-off time and is charged and discharged through a key switch circuit.